While a number of recent efforts are being made to achieve a finer pattern rule in the drive for higher integration and operating speeds in LSI devices, DUV and VUV lithography is thought to hold particular promise as the next generation in microfabrication technology. In particular, photolithography using an ArF excimer laser as the light source is thought requisite to the micropatterning technique capable of achieving a feature size of 0.13 μm or less.
The ArF lithography started partial use from the fabrication of 130-nm node devices and became the main lithography since 90-nm node devices. Although lithography using F2 laser (157 nm) was initially thought promising as the next lithography for 45-nm node devices, its development was retarded by several problems. A highlight was suddenly placed on the ArF immersion lithography that introduces a liquid having a higher refractive index than air (e.g., water, ethylene glycol, glycerol) between the projection lens and the wafer, allowing the projection lens to be designed to a numerical aperture (NA) of 1.0 or higher and achieving a higher resolution. See Journal of Photopolymer Science and Technology, Vol. 17, No. 4, p 587 (2004).
In the photolithography using an ArF excimer laser (wavelength 193 nm) as the light source, a high sensitivity resist material capable of achieving a high resolution at a small dose of exposure is needed to prevent the degradation of precise and expensive optical system materials. Among several measures for providing high sensitivity resist material, the most common is to select each component which is highly transparent at the wavelength of 193 nm. For example, polymers of acrylic acid and derivatives thereof, norbornene-maleic anhydride alternating copolymers, polynorbornene, ring-opening metathesis polymers, and hydrogenated ring-opening metathesis polymers have been proposed as the base resin. This choice is effective to some extent in that the transparency of a resin alone is increased.
Studies have also been made on photoacid generators. In prior art chemically amplified resist compositions for lithography using KrF excimer laser, photoacid generators capable of generating alkane- or arene-sulfonic acids are used. However, the use of these photoacid generators in chemically amplified resist compositions for ArF lithography results in an insufficient acid strength to scissor acid labile groups on the resin, a failure of resolution, or a low sensitivity. Thus these photoacid generators are not suited for the fabrication of microelectronic devices.
For the above reason, photoacid generators capable of generating perfluoroalkanesulfonic acids having a high acid strength are generally used in ArF chemically amplified resist compositions. These photoacid generators capable of generating perfluoroalkanesulfonic acids have already been developed for use in the KrF resist compositions. For instance, JP-A 2000-122296 and U.S. Pat. No. 6,048,672 (or JP-A 11-282168) describe photoacid generators capable of generating perfluorohexanesulfonic acid, perfluorooctanesulfonic acid, perfluoro-4-ethylcyclohexane-sulfonic acid, and perfluorobutanesulfonic acid. JP-A 2002-214774, US Patent Application Publication 2003-0113659 A1 (JP-A 2003-140332), and US Patent Application Publication 2002-0197558 A1 describe novel acid generators capable of generating perfluoroalkyl ether sulfonic acids.
Among these, perfluorooctanesulfonic acid and homologues thereof (collectively referred to as PFOS) are considered problematic with respect to their non-degradability and biological concentration in the environment. Manufacturers made efforts to develop partially fluorinated alkane sulfonic acids having a reduced degree of fluorine substitution as the replacement to PFOS. For instance, JP-A 2004-531749 describes the synthesis of α,α-difluoroalkanesulfonic acid salts from α,α-difluoroalkene and a sulfur compound and discloses a resist composition comprising a photoacid generator which generates such sulfonic acid upon exposure, specifically di(4-tert-butylphenyl)iodonium 1,1-difluoro-2-(1-naphthyl)-ethanesulfonate. JP-A 2004-2252 describes the development of α,α,β,β-tetrafluoroalkanesulfonic acid salts from α,α,β,β-tetrafluoro-α-iodoalkane and sulfur compound and discloses a photoacid generator capable of generating such a sulfonic acid and a resist composition comprising the same. JP-A 2002-214774 discloses such photoacid generators as difluorosulfoacetic acid alkyl esters and difluorosulfoacetic acid amides although their synthesis method is lacking. Furthermore, JP-A 2005-266766 discloses a photosensitive composition comprising a compound capable of generating a partially fluorinated alkane sulfonic acid having a sulfonylamide structure derived from perfluoroalkylene disulfonyl difluoride.
In an attempt to form a fine feature size pattern having a pitch of less than 200 nm, however, side-wall roughening (known as line edge roughness, LER) of the resist pattern becomes noticeable even on use of these acid generators. It is pointed out that with a progress of miniaturization, the edge roughness of a resist pattern after development is reflected by the edge roughness after etching, which is detrimental to device characteristics. In order to reduce the edge roughness of resist, many improvements have been proposed on the polymer side including optimization of polymer molecular weight, narrow dispersity polymers, living anion polymerization or living radical polymerization for the polymerization of such polymers, a dropwise polymerization technique for preventing variation of copolymer composition, and prevention of swelling during development by introducing recurring units having a hexafluoroalcohol group as the adhesive group. Another effective means for reducing the resist edge roughness is by increasing the amounts of an acid generator and a quencher (or basic compound) to enhance the contrast. The increased amount of acid generator, however, reduces transparency so that the resist profile is tapered or increases the acid diffusion distance to give rise to problems such as a loss of exposure margin and an increased mask error factor (MEEF).
Under the circumstances, it was proposed to form a polymer from an acryloyloxyphenyl diphenyl sulfonium salt as a monomer (as described in JP-A 4-230645) and to incorporate this monomer into a polyhydroxystyrene resin for improving the LER of the base resin (as described in JP-A 2005-84365). These polymers having acid generating units bonded to the main chain are effective in improving resolution and LER to some extent, but are not regarded as exerting satisfactory performance. To meet the demand for further miniaturization of the pattern rule, further improvements in resolution and LER are desired.